GLOBAL WARMING: Scientific Background

Illustration of the Earth’s radiative balance. Note that the amount of radiation reflected by the Earth and its atmosphere to space is about 30% of the total light that reaches Earth. (Adapted from: NOAA)
To understand the scientific issues, we need to acquire some scientific background. The principles of the "greenhouse effect" are not difficult to understand. The effect is part of the “radiation balance” of the Earth. To keep from overheating, Earth re-radiates all the heat received from the Sun. This situation can be sketched as follows:

Sun’s radiation (mostly visible light) → absorbed and converted to heat → re-radiated to space (infrared)

Greenhouse gases interfere with the re-radiation to space. Given the nature of our atmosphere, energy from the Sun readily penetrates into the lower atmosphere and onto the surface of Earth and is converted to heat, but then cannot freely leave the planet. So the lower atmosphere is warmer with these gases than it would be without them. This can be sketched as follows:

Sun’s radiation → absorbed and converted to heat → some re-radiated to space (infrared) & some trapped by the atmosphere

In fact, the Earth re-radiates heat from the atmosphere, to keep the heat balance. The lower atmosphere is below the main level of re-radiation (about 5 km), thanks to the greenhouse effect. Thus, the presence of certain “greenhouse gases” that trap heat (water vapor, carbon dioxide, methane, nitrogen oxide, and CFCs) allows the lower atmosphere to stay warm by shifting the main level of re-radiation upward. By increasing the abundance of these gases in the atmosphere, humankind is increasing the overall warming of the Earth’s surface and lower atmosphere, a process called "global warming." The figure below illustrates the radiation balance and the role of greenhouse effect.

Feedback loop involving water vapor in the atmosphere. Increased temperature leads to increased water vapor in the atmosphere. Because water vapor is a greenhouse gas, increased water vapor in the atmosphere leads to further warming. This is an example of a positive feedback.

However, cloud formation also increases with increased water vapor. This brings a more complex feedback into play and may stop the loop from running wild.
Changes in climate involve a large number of elements of the Earth’s system, including: All of these effects "feed back" into the climate system and change the amount of warming. This change in warming then changes the system some more, which then changes the warming some more and so on. These sorts of loops whereby the warming of the Earth and the elements that cause warming (or cooling) interact with one another are called “feedbacks.” Feedbacks can be either positive or negative. Positive feedbacks act to increase the original change, that is, they destabilize the Earth’s climate. Negative feedbacks act to minimize change and thus stabilize the climate. Most of the attention is given positive feedbacks since they are the ones causing trouble, potentially. Multiple climate feedback systems are exceedingly difficult to handle even when using a very large computer and even when very clever physicists program the computer. What the computer results do show us is that we get increased warming when putting carbon dioxide in the air, and we should expect roughly the amount of warming we have observed so far, considering the various types of gases and particles we put in the air, from driving automobiles, producing energy, growing rice and even raising cattle.